In electrical discharge machining, a tool electrode is spacedly juxtaposed with a workpiece to form a machining gap therewith flooded with a machining liquid of dielectric nature, and a succession of electrical pulses are applied across the machining gap to produce discrete time-spaced electrical discharges between the tool electrode and the workpiece through the liquid medium. Each electrical discharge, when fired or triggered at the machining gap regulated as to appropriate dielectric and conductivity conditions, results in stock removal from the workpiece. A succession of such electrical discharges may occur at the machining gap held under an optimum state to cumulatively remove material from the workpiece. As material removal proceeds, the tool electrode is displaced relative to the workpiece generally to maintain the gap spacing substantially constant. In three-dimensional or sinking-type EDM, the shape and the path of displacement of the tool electrode determine the configuration of a cavity formed in the workpiece.
In wire-cutting EDM, the tool electrode is a continuous wire composed of, say, copper or brass and having a thickness ranging generally between 0.05 and 0.5 mm.phi.. The wire electrode is continuously displaced axially through a cutting zone in which the workpiece is disposed. The cutting zone is defined generally between a pair of wire guides arranged for supporting the wire electrode tightly therebetween so that the wire electrode linearly travels through the cutting zone while traversing the workpiece. The machining liquid is commonly a water medium of a specific resistivity ranging between 10.sup.2 and 10.sup.5 ohm-cm and continuously supplied into the machining gap defined between the traveling wire electrode and the workpiece. A contouring feed is effected between the traveling wire electrode and the workpiece transversely to the wire axis along a prescribed path so that a desired contour of the shape determined by the path is formed in the workpiece.
One of the power supplies for providing machining pulses, which has been recognized to be suitable in the art of EDM and which has been found to be particularly advantageous in wire-cutting EDM, is a capacitor-type pulse generator in which a capacitor is connected on one hand in series with a DC source and is connected on the other hand across the machining gap defined between the electrode and the workpiece. The capacitor is periodically charged by the output voltage of the DC source, the charge stored on the capacitor in each charging cycle being impulsively discharged through the machining gap, thereby forming an individual machining pulse. A periodically operated switch may be included in the circuit connecting the DC source to the capacitor to insure the periodicity of charging without fail. A further switch may be included also in the circuit connecting the capacitor to the machining gap and turned on when the charging voltage on the capacitor reaches a predetermined level so that the occurence of any incomplete or premature discharge to the machining gap may be avoided.
One of the most important problems peculiar to wire-cutting operations is that of wire breakage which should be minimized. The breakage most often occurs when machining pulses of excessive energy are supplied to the machining gap. The approach taken by the prior art for wire-cutting EDM operation with a capacitor-type power supply as described to avoid wire breakage is therefore to limit the output voltage of the DC source to a sufficiently low level so that no such voltage buildup develops on the capacitor and the machining gap that may result in a gap discharge of excessive energy, i.e. an impulse of excessive peak current and duration. This approach, however, entails an undesirable increase in the capacitor charging time in each charging cycle and an excessive decrease in the discharge energy, hence a significant drop in the discharge repetition rate and mean machining current. As a consequence, the rate of material removal has been undesirably low.